Water-based coating mixture, method for application of corrosion protection layer with said mixture, substrates coated thus and use thereof

ABSTRACT

The invention relates to a mixture for the application of a polymeric coating to a substrate. Said mixtures comprises water and optionally an organic solvent, at least one water-dispersible radically polymerizable compound, at least one water-dispersible polymerizable crosslinking compound and at least one water-dispersible compound which forms free radicals upon exposure to actinic radiation. The radically polymerizable compound cannot fully set with the free radicals formed on irradiation at room temperature, but rather comprises further reactive bonds, which lead to further or complete setting with the crosslinking compound. The invention further relates to a method for application of a non-adhesive corrosion protection layer to a substrate, by application of a mixture to the surface of a metallic or polymeric substrate, drying the mixture and irradiation of the applied coating with actinic radiation of such an intensity and for such a period that a solid, hard, tough coating is formed.

The present invention relates to a mixture for the application of apolymeric coating to a substrate, wherein the mixture contains water andoptionally also organic solvent as well as at least one free-radicallypolymerisable compound. The invention relates in particular to a mixturefor the application of a UV-hardenable, weldable anticorrosion layer toa metallic substrate, a process for the application of a slideable(non-friction) anticorrosion layer, as well as a substrate coated inthis way, in particular a metal sheet.

Weldable protective coatings of the aforementioned type based oninorganic pigment particles and organic polymers are known and aredescribed for example in DE-C 34 12 234 for electrolyticallythin-galvanised, ductile, phosphated and chromated steel sheet. Theanticorrosion primer consists of a mixture of 70% to 95% zinc,aluminium, graphite and/or molybdenum sulfide and anticorrosion pigmentsas well as 5% to 30% of an organic binder and optionally additives,referred to the dry film. The organic binder is based on polyester resinand/or epoxide resin.

EP-B 0 298 409 discloses such coatings for steel sheet, which comprise alayer of pyrogenic silicic acid and a hardened organic matrix that hasbeen obtained from an epoxide resin and a polyvalent isocyanate bythermal crosslinking. This coating is applied to an insoluble chromatefilm that in turn is applied to the steel coated with a layer based onaluminium and/or zinc.

EP-B 0 344 129 discloses similar coatings with a thickness of up to 2μm, which have been obtained by the hardening of epoxide resins havingan epoxy equivalent between 500 and 2000 by means of amines, melamines,phenolic resins and/or the like.

In EP-A 0 761 320 coated steel sheets are described that carry anorganic protective layer that has been produced by electrolyticpolymerisation of ionogenic polymerisable organic compounds from aqueoussolution.

In EP-A 0 659 855 an aqueous coating mixture is described from whichhardenable rust prevention coatings can be deposited. The mixturecontains at least one water-soluble alkyd resin and at least onewater-soluble acrylic resin, which in each case has an acid value of 25to 100, a hydroxyl value of 35 to 200 and an SP value of 10 to 11, aswell as a hardener selected from oxazoline compounds and melamine resinsand at least one acidic catalyst.

All these known coating mixtures are thermally hardened, a permanentresistance to chemicals and weathering influences as well as asufficient rust prevention being achieved. The thermal hardening has thedisadvantage of a high energy consumption. The temperature of thesubstrates (=supports) is for the hardening generally in the range from140° to 250° C.

On account of the high temperatures during hardening the coatingsproduced in this way are less elastic than those that have beensubjected to lower temperatures. Accordingly the substrates, such as forexample steel sheets, that have been provided with an organic coatingthermally hardened at high temperature are ductile only to a limitedextent, for example by deep drawing or edging. For this purpose atreatment with drawing oil is generally necessary, which could howeverbe omitted if the coatings had a higher elasticity. The necessary highhardening temperatures may also lead to structural changes in thesubstrate.

The known coating mixtures often contain zinc powder. Such mixtures tendto undergo corrosion, which occurs between the pigmented layer and themetallic, optionally galvanised substrate. In addition, in order toachieve a weldable coating for electric welding a content ofelectrically conducting constituents is necessary, which ensures theminimum electrical conductivity necessary for welding.

WO 00/75250 in the name of the Applicants discloses a UV-hardenablecoating mixture with anticorrosion properties. The mixture is suitablein particular for the coating of steel strip or sheeting that has beengalvanised and/or chromated. Such materials are generally used in theautomobile industry. The coating mixture contains a polymeric organicbinder, a low molecular weight, liquid, free-radically polymerisablecompound, a compound forming free radicals under the action of actinicradiation and, as conducting pigment, an oxide, phosphate and/orphosphide of iron or aluminium or graphite-mica pigments. The binder isselected from the group comprising condensation resins, epoxy resins,poly(methyl)acrylates, polyurethanes, polyesters and polyethers,preferably epoxidised novolaks, bisphenol-epichlorohydrin condensationproducts and esterification products of these resins or polymers with(meth)acrylic acid. The free-radically polymerisable compound is amixture of compounds, at least some of which contain more than onepolymerisable group in the molecule, or consist wholly of these,preferably esters of an α, β-unsaturated carboxylic acid, preferablyacrylic acid or methacrylic acid, with a dihydric or polyhydricmonomeric or oligomeric alcohol. The mixtures are preferably free oforganic solvents and water. The organic coatings that are obtained canreadily be formed and welded after the UV hardening, and provide anoutstanding corrosion protection.

They have the disadvantage however that the hardening has to be carriedout largely or even wholly with actinic radiation. This requires—inparticular for strip plants that operate at high speed—a very highexpenditure on plant technology and energy use. At the present time thespeed of strip plants in which the hardening is to be carried outlargely or wholly by UV radiation is restricted to speeds of up to about60 or up to about 80 m/min. The investment expenditure is extremelyhigh.

Whereas the steel sheets for the coating of which the mixtures describedhereinbefore have been developed have up to now generally been chromatedin order to achieve a better corrosion resistance, this procedure isincreasingly being discontinued for environmental reasons. It is assumedthat sheets that have been pretreated in a chromium-free manner will beused practically exclusively in the near future, at least in theautomobile industry.

Such non-chromated metallic substrates, for example steel strip or steelsheets, require in some cases, as has now been shown, a relatively highthickness of the layer applied thereto in order to ensure the samecorrosion resistance as in chromated sheets. The dry film layerthickness of the polymeric coating that is applied to the pretreatmentcoating must then according to the prior art have a thickness in therange from 4 to 10 μm, whereas the coating in the case of chromatedsubstrates according to the prior art need only have a thickness in therange from 2 to 4 μm.

The coating mixtures available up to the present time are however oftenformulated for the layer thicknesses necessary in the case of chromatedmetallic substrates. In this connection, having regard to weldability,the amount of conducting pigment that is present is in general alsoappropriately adapted to the layer thicknesses required for chromatedsteel. The conductivity can of course in principle be adapted to therequirements pertaining to high layer thicknesses simply by adding alarger amount of pigment. Cases arise however, in particular with regardto the composition and properties of finely matched coating mixtures, inwhich the amount of pigment required to achieve the desired weldabilitycan no longer be added. The reasons for this may be a viscosity thatrises to too high values, a decrease in the reactivity, or theoccurrence of inhomogeneities up to the formation of agglomerates. Insuch cases a change in the composition of the mixtures is oftenunavoidable.

There is thus a need for coating mixtures which, apart from the desiredproperties of UV-hardenable coating mixtures outlined above, enable highlayer thicknesses combined with good weldability to be achieved withoutthe composition possibly having to be altered.

Furthermore in particular coating mixtures are desired by steelmanufacturers that permit a complete treatment and coating of the steelor steel sheet subsequently delivered to the further processing plants,without interruption in a production unit. Up to now the steel stripsand sheets are generally galvanised and if desired chromated at themanufacturer's after completion of the rolling process, and are thenwound into rolls. The rolls of metallic strip obtained in this way(coils) are then transported to the coating unit, where thepolymer-containing coating is applied. The transportation to the coatingunit as well as the uncoiling and coiling of the roll representundesirable cost factors that should be avoided.

A UV-hardenable coating mixture is desired that enables the pretreatmentand the coating with a polymer-containing coating to be-carried out in asingle production unit (so-called inline process). For this purposespecial requirements are placed on the hardenability of the coatingmixture. The galvanising of a steel sheet is generally carried out atspeeds starting from ca. 120 m/min. In order to ensure a problem-freeprocedure without intermediate storage, the coating including thehardening must likewise be executed at such speeds. For this reasoncoating mixtures are required that permit a coating at these high speedsand in which the resultant layer has a sufficient hardness.

The object of the present invention is to provide coating mixtureshaving the properties mentioned above as regards weldability andhardenability. Furthermore these coating mixtures should also besuitable for application in high-speed strip plants. The coatingmixtures should in addition be free as far as possible of heavy metalsand should be able to be applied to pretreatment coatings that are asfar as possible free of chromium. Finally, the object of the inventionis to provide a suitable process for the application of such coatingmixtures that permits the outstanding properties to be achieved in areliable and reproducible manner.

This object is achieved by a mixture for the application of a polymericcoating to a substrate, wherein the mixture contains water andoptionally also organic solvent as well as at least onewater-dispersible, free-radically polymerisable compound, at least onewater-dispersible polymerisable post-crosslinking compound and at leastone water-dispersible compound that forms free radicals under the actionof actinic radiation, wherein the free-radically polymerisable compoundmay not harden completely at room temperature with the free radicalsformed under the action of the actinic radiation, but also containsreactive bonds that in contact with the post-crosslinking compound leadto a major or complete hardening.

This object is also achieved by a mixture for the application of ananticorrosion layer to a metallic substrate, said mixture containingwater and optionally also organic solvent, at least one free-radicallypolymerisable binder, at least one compound that forms free radicalsunder the action of actinic radiation, at least one electricallyconducting substance such as for example a pigment, and at least onepost-crosslinking compound.

Under the action of actinic radiation the at least one photoinitiatorforms free radicals that react with the free-radically polymerisablebinder and incompletely crosslink the latter during and possibly shortlyafter the so-called UV hardening. After this chemical reaction thisbinder may still contain reactive groups such as OH and polyol groups,which may limit the chemical resistance and the corrosion resistance ofthe coating formed therefrom.

It has now been found that it is very advantageous if there is added tothe binder mixture a post-crosslinking compound that after a UVhardening saturates the bonds, not all of which are free, and permits asubsequent chemical hardening to effect a major or completecrosslinking. This frequently occurs through polyaddition. Thepost-crosslinking compound may largely or wholly chemically harden thefree-radically polymerisable binder, which is only partially hardened,in that it bonds with the reactive groups of the UV-hardened binder andthereby significantly increases the corrosion resistance of the hardenedpolymer.

This object is furthermore achieved by a process for the application ofa slideable anticorrosion layer to a substrate, in which a mixture isapplied to the surface of a metallic or polymeric substrate, preferablyto an uncoated or coated steel strip or steel sheet or to a plasticspart, wherein the mixture is dried and the applied coating is irradiatedfor a sufficient time with actinic radiation of such an intensity that asolid, hard, viscoplastic coating is formed.

This solid, hard, viscoplastic layer normally also becomes slightly moresolid and slightly less viscoplastic in the post-crosslinking. Thiscoating can also often still be shaped, even if in some cases with alesser shaping intensity. Due to the post-crosslinking this coatinghowever acquires in particular a better resistance to organic solvents,to dissolved alkalis and other aggressive chemical substances, as wellas a significantly improved corrosion resistance.

The term “water-dispersible” within the context of the present inventionis understood to denote everywhere in this text the generic term“water-dispersible”, “water-soluble”, “water-miscible” and“water-dilutable”. Preferably substantially all organic compounds thatare added to the mixture or that are contained in this mixture beforethe crosslinking are water-dispersible. Preferably only or virtuallyonly the electrically conducting compounds and possibly anticorrosionpigments as well as possibly individual further additives are notdispersible in water according to the generic term defined above.

A very wide range of metallic and polymeric substrates can be coatedwith the coating mixture according to the invention containing water andoptionally organic solvent(s) and with the process according to theinvention, namely metal sheets, panels, strip materials, profiles, cutsections, wires, moulded articles, in each case in the uncut orcut/stamped state and in the unformed or deformed/formed state, beforeand/or after joining, for example by clinching, bonding, welding, etc.This coating mixture and this process are particularly preferred forstrip materials and wires that are to be coated at high speeds since apartial crosslinking can be achieved very quickly in this way.

As metallic substrates in principle all types of metallic materials aresuitable, in particular steels, stainless steels, metallic materialsprovided with zinc-containing and/or aluminium-containing coatings,aluminium and aluminium alloys, magnesium alloys, magnesium alloys,copper and copper alloys, brass, bronze, nickel alloys, titanium alloys,zinc alloys and tin alloys.

In principle all plastics surfaces, sheet-type surfaces and paintsurfaces are suitable as polymeric substrates. These include for exampleplastics panels, plastics moulded parts, panels and moulded parts, inparticular for furniture, in building and construction, in apparatusfabrication, in vehicle production and in the aerospace industry. Theelectrically conducting coating according to the invention may in thisconnection be used, in particular after earthing the coating, to preventa build-up of electrostatic charge on the polymeric components orsurfaces.

The coating procedure with the polymer-containing coating mixtureaccording to the invention preferably takes place after a pretreatmentwith an aqueous composition largely or wholly free of chromium (VI)compounds, particularly preferably also largely or wholly free ofadditives containing heavy metals, such as for example those based oncadmium, cobalt, copper and/or nickel.

The polymer-containing coating according to the invention is alsopreferably largely or wholly free of chromium (VI) compounds,particularly preferably also largely or wholly free of additivescontaining heavy metals, such as for example those based on cadmium,cobalt, copper and/or nickel.

The coating mixture according to the invention contains water. The wateris added to the mixture preferably in the form of an aqueous binderdispersion and/or in the form of fully deionised water. Thefree-radically polymerisable binder is contained dissolved, dilutedand/or dispersed in the mixture. The mixture may also contain at leastone organic solvent, preferably in amounts of not more than 30 wt. %,particularly preferably of not more than 15 wt. %, referred to theamount of water. The at least one organic solvent that is used shouldeither be completely soluble in water or water-miscible orwater-dilutable. Preferred organic solvents are monohydric or polydricalcohols, esters, glycol ethers and ketones.

The composition according to the invention contains at least one organicbinder, in which connection however at least one of the binders must befree-radically polymerisable. If only one organic binder is contained,then the post-crosslinking compound is also contained in capped form. Ifthe post-crosslinking compound is present not in capped form, it isregarded as organic binder. The content of the organicbinders—calculated as dry substance in the overall water-containingcomposition—may lie in the range from 8 to 50 wt. %, preferably in therange from 15 to 35 wt. %. The proportion of the free-radicallypolymerisable binders—optionally as part of the organic binders andcalculated as dry substance in the overall water-containing composition(wet paint)—may lie in the range from 8 to 30 wt. %, a minimum contentof 10 wt. % and a maximum content of 25 wt. % being preferred. A minimumcontent of 12 wt. % and a maximum content of 22 wt. % are particularlypreferred.

The weight ratio of the free-radically polymerisable binders to thepost-crosslinking binders is preferably in the range from 40:60 to 95:5,particularly preferably in the range from 60:40 to 85:15, referred tothe dry weights. The proportion of UV hardening to chemicalpost-crosslinking is in the same relationship.

The free-radically polymerisable compounds contain unsaturatedpolymerisable groups that react with the groups formed from thephotoinitiators under the action of radiation and that can form awater-insoluble network. The network formed in this way and its waterinsolubility are also to a certain extent important for the corrosionresistance, in particular to water and to aqueous salt solutions. Thebinder itself is preferably at least one oligomer, pre-oligomer,polymer, pre-polymer, copolymer or block copolymer. The aforementionedpolymerisable groups are then present on the latter and permit furtherpolymerisation with the groups of the post-crosslinking compound. Theterm “polymer” within the context of the present invention includes thestructural forms listed for the binders and mentioned above.

The mixture according to the invention contains at least onefree-radically polymerisable compound that comprises free OH and/orpolyol groups. Suitable free-radically polymerisable binders areacrylates, methacrylates, epoxy resins, polyurethanes and polyestersdispersible in water, preferably epoxy acrylates, urethane acrylates andacrylic-containing polyesters, in particular epoxy acrylates, urethaneacrylates and acrylic-containing polyesters that comprise free OHgroups.

The polymerisable groups present in the added free-radicallypolymerisable binder are preferably ethylenically unsaturated,methylenically unsaturated and/or propylenically unsaturated groups. Inorder to achieve as good a crosslinking as possible and thusinsolubility and resistance of the layer to solvents, chemicals andinfluence of weathering, at least part of the binder used according tothe invention should contain at least two polymerisable groups.Preferred ethylenically unsaturated groups are derived from α-,β-unsaturated carboxylic acids. These carboxylic acid(s) may be boundfor example as ester to the binder. Preferred α-, β-unsaturatedcarboxylic acids are acrylic acid(s) and methacrylic acid(s).

Such oligomers and polymers are in principle known to the person skilledin the art, in particular as regards the free-radically polymerisablebinders and the post-crosslinking compounds, as are the oligomers orpolymers similar thereto or derived therefrom, which may likewise beused in the process according to the invention. The oligomers mayalready be present in liquid form without the addition of water and/ororganic solvent and contain, apart from the resin, also plasticisers,while the polymers are often present in solid form as substantiallypure, in many cases soft resin.

The coating mixtures according to the invention may be one-componentmixtures, in other words they contain only a binder and are thus presentin the form of a mixture of all the necessary constituents if thepost-crosslinking compound is capped. It is preferred if the coatingmixtures according to the invention are available in the form oftwo-component mixtures, in other words containing two different binders.The first component (A) may contain all or almost all constituents, forexample except for the post-crosslinking compound (free-radicallypolymerising binder, etc.); the second component (B) may then containthe post-crosslinking compound. The component B is preferably introducedinto the component A by stirring, before the processing, and isintimately mixed with the latter, for example by stirring. Three or morecomponents may however also be combined in a similar way.

Preferably the coating mixture is chosen and the process according tothe invention is adjusted so that the so-called UV hardening andpossibly also at the same time a chemical post-crosslinking areinitiated by the actinic radiation. In this application, for the sake ofsimplicity actinic radiation is termed UV radiation and the crosslinkingresulting therefrom is termed UV hardening, since in most cases actinicradiation is the main component of the radiation. Whereas UV hardeningunder powerful UV radiation leads in 0.5 to 100 seconds, preferably in0.8 to 60 seconds and in particular in 1 to 5 seconds, to a majorcrosslinking of the UV-hardenable constituents, the chemicalcrosslinking that may possibly simultaneously occur requires a very muchlonger time, generally hours or up to ca. 10 days depending on theproportions and types of the crosslinkable compounds, layer thicknessesand temperatures employed, in order to achieve a major to completecrosslinking of all the crosslinkable compounds. The chemicalpost-crosslinking can be accelerated or intensified by heating. Sincecoated steel strips are often coiled in the hot state at a temperatureof about 40° C., the heat in the coil is stored for an even longer timeand may lead to a quicker and to a more complete post-crosslinking. Thechemical crosslinking that is possibly accelerated or intensified byelevated temperatures is referred to as post-crosslinking in the presentApplication.

If the post-crosslinking compound is present in capped form, it mustfirst of all be heated to a temperature above the capping temperature,which mainly lies in the range from 75° to 150° C., in order to bringthe capped compound into a reactive state. By means of the capping it ispossible to prepare a paint that is not permanently crosslinking and isstable on storage, which without capping would already react furtherwithin a few hours at room temperature. At the desired point in time thecapped, post-crosslinking compound can be activated by heating at leastto the capping temperature and the post-crosslinking can be initiated.

The post-crosslinking binders may be those based on aliphatic and/oraromatic isocyanate, in particular diisocyanate or polyisocyanate orcyanurate, wherein the isocyanate or isocyanurate terminal groups mayoptionally be blocked (=capped) by a blocking agent known per se.Depending on their nature, the blocking agents may be caused todissociate at specific minimum temperatures approximately for example inthe range from 75° to 160° C., so that the dissociated reactive groupscan react further. This can serve for the intermediate storage ofUV-hardened substrates such as for example coils before furtherprocessing, before further processing after forming, or before paintingover, bonding and/or welding.

The proportion of UV hardening in the overall hardening/ crosslinking ispreferably in the range from 90% to 30%, particularly preferably in therange from 85% to 35% and most particularly preferably in the range from75% to 45%. The remaining proportion of the crosslinking is chemical innature, even if it can be thermally intensified or accelerated.

If the post-crosslinking compound(s) are all present in capped form,then post-crosslinking cannot be started by the UV radiation, since forthis purpose the heating is not sufficient to reach the cappingtemperature. Accordingly it is possible in automated production linesand on high-speed conveyor belts to apply coatings according to theinvention that are first of all hardened by UV radiation but arenevertheless present incompletely crosslinked. The coated substrates canbe stored in this state, or may be followed by further process steps,such as for example a forming, since the UV-hardened coating may be soflexible that it can be deformed without damaging the polymeric coating,also when lying on a metallic substrate. The coated substrates maythen—for example after storage or forming—be heated at least to thecapping temperature, so that the post-crosslinking compounds becomereactive and the chemical post-crosslinking is initiated.

An important component of the coating mixture according to the inventionis the at least one post-crosslinking compound. This serves to achieve amajor or even complete hardening of the mixture according to theinvention after the UV hardening. On account of relatively highproportions of this post-crosslinking it is possible to increasesignificantly the strip speed during UV hardening, since only a minorproportion of all crosslinkable compounds are to be hardened and sincethe proportion of the crosslinkable compounds in the coating layeraccording to the invention is relatively slight. In general the stripspeed when changing over from an (almost) pure UV hardening to a mixedcrosslinking according to the invention can be increased by ca. 10 to70%, preferably by ca. 20 to 50%, in particular by ca. 30 to 40% with aconstant lamp output.

It is therefore possible to effect only part of the crosslinking by UVhardening and subsequently to effect a gradual post-crosslinkingpossibly extending over about 12 days. The post-crosslinking may extendover several days at room temperature and may be accelerated by elevatedtemperature and/or in the presence of a catalyst for thepost-crosslinking compound, such as for example dibutyltin laurate(DBTL). The post-crosslinking is initiated in the case ofpost-crosslinking compounds present in uncapped form by the input ofheat during the actinic irradiation, since the polymeric layer isthereby heated to temperatures in the range from 60° to 95° C., inparticular in the range from 70° to 85° C.

The post-crosslinking compound effects a post-crosslinking of thecoating mixture by a thermally initiated reaction. After the coating ofthe metallic substrates followed by a heat treatment in order toevaporate the water and by the UV irradiation, the metallic substrategenerally has a temperature of up to 80° C. The post-crosslinking thentakes place.

The at least one post-crosslinking compound is generally selected sothat this can react with compounds or functions that are present in thepolyol-based binder used according to the invention and can be activatedby isocyanate or isocyanurate. As polyols there are preferred compoundsbased on acrylate, methacrylate, epoxide, polyurethane and polyester, inparticular epoxy acrylates, urethane acrylates and acrylic-containingpolyesters. The compounds or functions may also be those that arepresent in the binder mixture according to the invention, for example inorder to establish a desired solubility or viscosity. The compounds orfunctions may also in addition be added to the coating mixture in orderto permit the crosslinking.

An important feature of the post-crosslinking compound is itssolubility, dilutability and/or dispersibility in water.

Examples of preferred post-crosslinking compounds include isocyanatesand isocyanurates. These may be based on 2,4-toluenediisocyanate or2,6-toluenediisocyanate (TDI), 4,4′-methylenedi(phenyl)isocyanate (MDI)or hexamethylenedi-isocyanate (HDI). Isocyanates and isocyanurates basedon HDI are preferably used. The post-crosslinking compounds react withthe free OH and polyol groups of the UV hardening resin with theformation of polyureas, which as is known are very stable compounds, andrelated chemical compounds.

An example of functions and compounds that react in thepost-crosslinking stage with the post-crosslinking compound usedaccording to the invention are hydroxyl groups, for example inpolyhydric alcohols, polyether alcohols or polyester alcohols. Thealcohols, in particular polyether alcohols and polyester alcohols, arepreferably of oligomeric and/or polymeric nature.

The hydroxyl groups are preferably chemically bound to the binder, forexample directly or via the organic skeleton of the aforementionedalcohols.

The hydroxyl contents of the free-radically hardening binders are often0.2 to 2% after the UV hardening. The molecular weights of thefree-radically hardening binder may, before the UV hardening, bepredominantly in the range from 500 to 20,000, and after the UVhardening predominantly in the range from 20,000 to 100,000 and afterthe post-crosslinking in most cases predominantly at least 100,000.

According to a preferred embodiment of the present invention a binderthat has available free hydroxyl groups is reacted with an isocyanate orisocyanurate as post-crosslinking compound. In particular anhydroxyl-group-containing polyurethane is used as binder. Thesepolyurethanes may, as previously mentioned, be present based on TDI,MDI, HMDI and/or HDI; they are preferably based on HDI.

The crosslinking according to the invention of the OH-containing binderswith the post-crosslinking compound also leads to an increase in thecorrosion protection of the obtained coating. The free hydroxyl groupson the other hand often have a corrosion-promoting effect. After thecrosslinking no free or virtually no free OH groups are present anylonger, and accordingly their corrosion-promoting action is eliminated.A larger proportion of OH groups could adversely affect the corrosionresistance to some extent.

Actinic radiation is understood to mean radiation whose frequencies orenergy are suitable or sufficient for the activation of thepolymerisation initiator (=photoinitiator). The radiation shouldnormally have at least the energy or the frequency of visible light orUV light. Short wavelength visible and/or ultraviolet light (UV light)is preferred. Any radiation of shorter wavelength, i.e. higher energy,is of course also suitable. Electron beam radiation for example may alsobe used in cases where the use of a photoinitiator is not necessary.

Suitable as compounds that form free radicals under the action ofradiation, in particular photoinitiators, are above all those compoundsthat exhibit a strong absorption in the spectral range of the employedradiation, in particular the near ultraviolet or short-wavelengthvisible light, i.e. approximately in the wavelength range from 180 to700 nm. Especially suitable are aromatic carbonyl compounds and theirderivatives, such as quinones, ketones and their ketals, for examplebenzyldimethyl ketal, benzoin, substituted benzoins and benzoin ethers,α-aminoketones; in addition polynuclear heterocyclic compounds such asacridines, phenazines and their substitution products as well assubstituted phosphine oxides such as for example bisacylphosphineoxides. Several photoinitiators may be added at the same time, which forexample specifically react at different wavelengths of the UV radiation.

A further important constituent is the at least one electricallyconducting substance, which is preferably water-insoluble, such as forexample inorganic pigment, in particular corrosion prevention or rustprevention pigment, for example in the form of oxides, phosphides and/orphosphates, in each case of aluminium, iron, copper, manganese,molybdenum and/or their mixtures or their combined compounds such as forexample double oxides and/or other electrically conducting pigments suchas for example graphite and/or graphite-mica pigments. Iron phosphideand/or magnetic iron oxide, in particular based on Fe₃O₄, are preferred.Graphite may in this connection also act as a dry lubricant. Magneticiron oxide has a very high electrical conductivity. The higher theelectrical conductivity of a pigment, the lower may be its proportion inthe coating mixture or in the polymeric coating, without reducing theweldability. The at least one electrically conducting substance, inparticular at least one pigment, may be used alone or as a mixture. Thepigment does not have to be coloured, grey or black, and may also becolourless or white or have a slight colour effect. The at least oneelectrically conducting substance is preferably present in the form ofparticles, in particular elongated or platelet-shaped particles,optionally in the form of crystals, crystal aggregates or agglomerates.The mean size of the particles before mixing into the coating mixture ispreferably in the range from 10 nm to 20 μm, particularly preferablyabove 30 nm and below 12 μm, most particularly preferably above 60 nmand below 10 μm, especially above 0.1 μm and below 8 μm. Even though thepreferred thickness of the hardened dry film layer is often only in therange from 3 to 9 μm, the electrically conducting particles may have amean particle size and/or a maximum particle size, in each case measuredunder a scanning electron microscope, that is greater than the averagedry film layer thickness of the hardened layer.

Instead of or in addition to at least one inorganic pigment there mayalso be used electrically conducting polymers, such as for examplepolyanilines.

Additives meeting a very wide range of tasks or having very widelydiffering compositions may be employed. The proportion of additivesapart from free-radically polymerisable binders, post-crosslinkingbinders, electrically conducting pigments, water and optionally organicsolvent may comprise 0.5 to 50 wt. % referred to the composition of thewet paint including the liquids, and may preferably comprise 2 to 20 wt.%.

Finely particulate powder qualities such as for example hydrophobicsilicic acid may serve to stabilise the suspension, which on account ofthe high pigment content can easily tend to settle out, and can help toflocculate floor sediment in vessels so that the floor sediment does notbecome encrusted and can easily be dissolved.

Additions of anticorrosion pigments such as for example based on oxide,phosphate and/or silicate or based on their alkaline earth, iron,manganese, silicon and zinc compounds, in particular based on calciumcompounds, may intensify the corrosion protection.

Additives such as for example those based on silane or siloxane mayimprove the surface quality of the coating since they promote theslideability of the coating. They may in addition intensify thehydrophobicity of the mixture and act as defoaming agents, in order toprevent the formation of micropores, and may thereby contribute to theformation of a coating that is as pore-free as possible together with acoating surface that is as pore-free as possible and thus sealed.

In order to prevent a premature polymerisation of the coating mixturesthese often contain small amounts of polymerisation inhibitors, forexample hydroquinone and its derivatives and tert.-butylphenols. Suchinhibitors are often already contained in the commercially availablepolymerisable compounds.

If the polymeric coating is not heated either during production orduring further processing and is also not heated in use to temperaturesabove about 50° C., above 80° C. or above 110° C.—the softening point ormelting point of the respective wax—and if the coating is not therebyadversely affected as regards the bondability for the processing use orintended use, then wax, in particular a polyethylene wax, may also beadded, which can serve as a forming aid.

The mixtures normally contain in addition coating auxiliaries, forexample surface-active substances, in particular polysiloxanes, silanesand/or silicon-free oligomeric and/or polymeric surfactants. They mayfurthermore contain defoaming agents, coupling agents, catalysts,corrosion inhibitors, pigments to increase the corrosion resistance andoptionally also dyes and/or dye pigments.

An addition of layer compounds such as for example layer silicates mayhelp to reduce or suppress the flow of hydrogen ions to the metallicsurface, which promotes the corrosion protection.

The quantitative proportions of the constituents of the coating mixturemay lie within the following ranges:

The coating mixtures may be wholly or largely anhydrous if they containan amount of organic solvent. If for example the mixture contains 1 or 5wt. % water, a content of organic solvent in the range from 20 to 40 wt.% is preferred. If for example the mixture contains 8 wt. % of water, acontent of organic solvent in the range from 15 to 35 wt. % ispreferred.

Water is contained in general in the range from 10 to 50 wt. %,preferably in the range from 15 to 40 wt. % and in particular in therange from 20 to 30 wt. %.

Organic solvent(s) is/are contained generally in the range from 0 to 50wt. %, preferably in the range from 1 to 30 wt. % and in particular inthe range from 2 to 12 wt. %, calculated here with reference to thecontent of water in the mixture.

Binder is generally contained in the range from 10 to 30 wt. %,preferably in the range from 15 to 25 wt. % and in particular in therange from 17 to 23 wt. %.

Electrically conducting substances such as for example pigment(s) is/aregenerally contained in the range from 30 to 75 wt. %, preferably in therange from 35 to 60 wt. %, in particular in the range from 40 to 48 wt.%. The contents of electrically conducting pigments may, like thecontents of those based on magnetic iron oxide, iron phosphide and/orgraphite, also be more than 50 wt. %. The contents of graphite and ironphosphide may preferably be 25 to 70 wt. %, particularly preferably atleast 28 up to 65 wt. % and especially at least 30.45 up to 62.5 wt. %.The contents of graphite, magnetic iron oxide and iron phosphide maypreferably be 25 to 70 wt. %, particularly preferably at least 30 or upto 65 wt. % and especially 35 to 62.5 wt. %. The ratio of the proportionof magnetic iron oxide to iron phosphide is preferably up to 15.4 wt. %.The ratio of graphite to iron-oxide is preferably up to 3.08 wt. %.

Post-crosslinking compound(s) is/are generally contained in the rangefrom 1 to 20 wt. %, preferably in the range from 1 to 10 wt. % or 2 to10 wt. %, especially in the range from 3 to 6 wt. %, in each case eitheronly uncapped or only capped post-crosslinking compounds.

Photoinitiator(s) is/are generally contained in the range from 1 to 60wt. %, preferably in the range from 2 to 24 wt. % and especially in therange from 2.5 to 3.5 wt. %. Wax(es) and/or wax-like substance(s) is/aregenerally contained in the range from 0 to 15 wt. %, preferably in therange from 0.5 to 10 wt. % and especially in the range from 1 to 6 wt.%.

Additive(s) is/are generally contained in the range from 0.01 to 5 wt.%, mostly in the range from 0.1 to 4 wt. %, preferably in the range from0.3 to 3 wt. %, particularly preferably in the range from 0.5 to 2 wt.%. Corrosion protection pigment such as for example silicate pigment ispreferably contained in an amount of up to 4 wt. %. The content ofstabiliser such as for example hydrophobic silicic acid is preferably upto 0.5 wt. %.

The weight ratio of the electrically conducting substance(s) to thebinders may be varied in particular in the range from 1:1 to 5:1,preferably in the range from 1.5:1 to 4:1. By increasing the proportionof the electrically conducting substance(s), which may vary within awide range, the weldability can also be improved very substantially. Inmany cases however an optimisation of various properties of theresultant dry film, such as for example weldability, hardness, strength,elasticity, chemical resistance, formability, sliding behaviour,bondability, corrosion resistance and paint adhesion is chosen that isappropriate for most or all properties.

The coating mixtures are generally produced by grinding or mixingwater-insoluble pigment particles with the remaining water-soluble,water-dilutable or water-dispersible constituents to form anhomogeneous, low viscosity composition. The grinding may be carried outas homogeneous intermixing, for example in a bead mill, in which forinstance a binder/water mixture, which may also contain organic solvent,is mixed with the pigments. The viscosity should be in a range thatpermits a uniform application to form a thin, for example 5 to 25 μm,preferably 6 to 22 μm and particularly preferably 10 to 20 μm thicklayer of the liquid coating mixture. When this liquid film is dried adry film layer is formed whose thickness is in the range from 0.5 to 15μm, preferably in the range from 1 to 10 μm, particularly preferably inthe range from 2 to 8 μm and especially in the range from 3 to 7 μm. Inthe determination of this layer thickness the tips of the pigmentparticles that often project from the polymeric composition are largelydisregarded. The dry film layer thickness is preferably determinedgravimetrically (layer weight determination) or by a β particleback-scattering method.

The binder mixture must have a viscosity suitable for coating. This isalso necessary in order to be able to apply uniformly the desired verythin coating layer thicknesses with as slight thickness variations aspossible. The viscosity of the coating mixture containing the particlesis preferably in the range from 100 to 1000 MPa·s at 25° C.

The viscosity can be adjusted by the choice of the type and amount ofparticularly the binder and water. The viscosity is generally in therange from 200 to 6000 MPa·s, in particular in the range from 500 to5000 MPa·s, measured at 25° C. without particle content. If theviscosity of the particle-free coating mixture is too high, the liquidfilm that is formed is too thick, whereas if it is too low the liquidfilm runs off and also cannot be properly applied with a so-called rollcoater.

The metallic substrate to be coated is preferably a strip or a sheet,which in many cases consists substantially of steel, galvanised orAlZn-coated steel or an aluminium alloy, and has a thickness of themetallic substrate in the range from 0.15 to 10 mm, especially fromabout 0.2 to 1.6 mm.

In particular a strip, a sheet or a panel is used that is suitable forthe production of for example automobile components. The coatingaccording to the invention may be applied in a conventional way. Forthis purpose a polymeric surface must be extremely clean. In additionthe metallic surface of for example a strip is normally for this purposeelectrolytically coated or hot-dip galvanised beforehand and thenpretreated with an aqueous pretreatment solution, wherein thispretreatment has hitherto frequently involved a chromating and/orphosphating. If no zinc-containing alloy is applied or if this metalliccoating is no longer freshly applied for the pretreatment, it isrecommended to carry out a cleaning, pickling and/or activation beforethe coating with the pretreatment solution. It is however preferred forthe pretreatment solution to be largely or wholly free of chromium. Itis furthermore preferred if the pretreatment solution is largely orwholly free of cobalt, copper and/or nickel or even free of all types ofheavy metals. Metallic substrates that have not been pretreated may alsobe employed. The weldable coating is then applied according to theinvention to this optionally pretreated surface.

As regards the points to be through-contacted in the welding, it shouldbe borne in mind that at least 2, 3 or even 4 polymeric coatings have tobe through-contacted in the weld contact. The thicker the individualpolymeric layers, the better the electrical conductivity of these layershas to be in order to permit a good through-contacting and ensure alonger service life of the welding electrodes. For this reason with forexample four polymeric coatings on two metal sheets it has a significanteffect on the electric resistance when welding whether the individualcoatings comprise a layer thickness of only 4 or of 8 μm. In additionthe metal sheets as well as the metallic coatings on the layers andpossibly also thicker pretreatment layers may contribute significantlyin increasing the electric resistance of the two superimposed coatedmetal sheets.

In general the strip or metal sheet is wound in the form of rolls,so-called coils. In order to apply the coating according to theinvention a coil is if necessary unwound and, after having been coated,is rewound again into a coil. Coatings are conveniently applied in acontinuous process. With the coating mixture according to the inventionit is however also possible to operate according to the so-called inlineprocess. The strip or metal sheet is normally electrolyticallygalvanised or hot-dip galvanised in the production line and is nextpretreated, for example chromated and/or phosphated, following which itcan then be coated according to the invention. The strip is wound into acoil only after it has undergone polymeric coating. Obviously anon-pretreated metallic material can also be coated according to theinvention.

In the coating process according to the invention the metallic strippasses for example through a coating station and then through at leastone hardening station. The coating may be effected by spraying, forexample through slit-shaped jets, by sprinkling, by dipping and/or byroller application. Roller coating is generally preferred. The polymericcoating is preferably carried out at room temperature or at atemperature slightly above room temperature, i.e. at temperatures in therange from about 20° to 40° C., the material and the substrate surfacepreferably having a temperature in the range from 20° to 30° C. Themixture is preferably applied so that the resulting dry film layerthickness is in the range from 3 to 12 μm, in particular in the rangefrom 4 to 10 μm.

After the coating the water can be evaporated in a circulating air oven,generally at a substrate temperature of 30° to 80° C., preferably 40° to60° C., which however depends in particular on the substrate thickness.The drying may be carried out for a time generally ranging from 1 to 30seconds, preferably from 3 to 20 seconds and particularly preferablyfrom 5 to 15 seconds, in order to allow the subsequent UV hardening. Ifthe strip is fed more quickly, then a higher substrate temperature isnormally necessary. The process according to the invention may onaccount of the only partial crosslinking with actinic radiation becarried out rather more quickly than is normally the case, for exampleat a rate of at least 75 m/min or even at least 125 m/min.

After the evaporation the hardening takes place, preferably duringpassage through a hardening station. There the strip is fed, optionallyunder an inert gas atmosphere such as for example nitrogen, at adistance of a few centimeters underneath a radiation source, whichpreferably corresponds approximately to the overall width of the strip.The strip speed may be chosen in particular depending on the layerthickness, light sensitivity of the layer, distance of the lamp from thelayer and lamp output. The speed may furthermore be dependent on whetherthe irradiation is carried out in air or alternatively in nitrogen. Thestrip speed may if desired be increased by installing two or moreradiation sources in series. Strip speeds of up to about 200 m/min canbe achieved with the mixture according to the invention. Preferred stripspeeds are currently generally in the range from 80 to 140 m/min.

UV light sources such as gas discharge lamps, xenon lamps or sodiumvapour lamps are preferably used as radiation sources, whose emissionmaxima lie in the spectral range from about 180 to 700 nm, in particularin the range from 200 to 600 nm, and which have a lamp output of 160 to240 watt/cm. Lamps that emit substantially in the short wavelengthvisible spectrum from about 400 nm to 550 nm may also be used. Inprinciple, radiation of higher energy, for example electron beamradiation, may also be employed for the hardening. The actinicirradiation, like the coating process, is preferably carried out atambient temperatures that are not or are not significantly above roomtemperature, i.e. generally not above about 50° C. The irradiated layersurface often reaches temperatures in the range from 60° up to about 80°C. due to the UV excitation, which includes an IR radiation component.

With substrate temperatures of in general 30° to 80° C., preferably 40°to 60° C., the post-crosslinking according to the invention additionallytakes place. Preferably the actinic hardening accounts for 40% to 80% ofthe crosslinking. Layers that are particularly anticorrosive, firmlyadherent and extremely formable are produced by the combination ofactinic hardening and the predominantly chemical post-crosslinking.

The layer composition and the hardening conditions should be chosen sothat a hard, solid, corrosion-resistant layer is formed, which howeveris sufficiently viscoplastic so that a forming of the substrate, forexample of the steel sheet, is possible without causing embrittlementcracks in the anticorrosion layer. The layer is outstandingly suitablefor bonding or welding to other parts. The coating mixture may bedesignated as a welding primer.

The production of the anticorrosion layer by the process according tothe invention permits a wide variation of the layer thickness within therange specified above. The layer adheres firmly and permanently to thesubstrate; it may be painted over in a conventional way, for example bycathodic dip coating, and has a smooth, slideable surface.

With a thickness of the hardened layer in the range from 3 to 12 μm,preferably in the range from 4 to 10 μm, more or fewer than about 1000welding points per electrode pair are obtained, depending on the natureof the metallic substrate, particularly in the welding region of about1.5 kA, depending on the nature of the metallic substrate, before theelectrode has to be replaced or re-milled. In this connection thehardened dry film should have an electrical resistance that is as far aspossible less than 250μΩ.

The corrosion-protected metal sheets obtained according to the invention(coil-coated steel) are then transported to the further processingplant, as a rule an automobile manufacturer, where they may betemporarily stored.

The metal sheets are uncoiled again and, depending on requirements, arestamped, formed and welded or bonded. On account of the properties ofthe coating mixture according to the invention a very good formability,both with as well as without the use of drawing oil, and a very goodweldability are achieved. After coating has been performed the parts areprocessed further as required, for example cleaned with an alkalineagent and further coated, for example by cathodic dip coating. Followingthis the metal sheet coated in this way or a subassembly coated in thisway can for example be treated with a filler and then painted with atopcoat.

The mixture according to the invention permits the incorporation of highpigment concentrations into the coatings obtainable therefrom. This isoften not possible with conventional polymeric mixtures at this stripspeed. Due to the large proportion of electrically conducting pigmentsor electrically conducting polymers, a good weldability is achieved alsowith high layer thicknesses. This is advantageous particularly whencoating chromate-free pretreated metallic substrates, for example steelstrips and sheets. The coating mixtures according to the invention aretherefore particularly suitable for the coating of steel strips andsheets that have been electrolytically galvanised or hot-dip galvanisedand preferably pretreated in a chromate-free manner, but have possiblyalso not been galvanised. Furthermore the mixtures according to theinvention permit, due to the post-crosslinking, a high strip speed inthe coating of the metallic substrate.

Due to the increased amount of a post-crosslinking instead of simplyactinic hardening the installation of additional UV lamps may also beunnecessary in the process according to the invention, which areotherwise often installed in order to achieve high or even higher stripspeeds. Large savings in investment costs can thereby be achieved. Also,the energy balance is more favourable on account of the content ofpost-crosslinking compounds. Despite a further increased strip speed,the process according to the invention can be operated withapproximately constant energy, material and process costs. Due to theincreased amount of a post-crosslinking instead of simply actinichardening, far larger dry film layer thicknesses can also bethrough-hardened by the process according to the invention, despite highstrip speeds. This is enhanced still further by the fact that theproportion of the oligomers of polymers that are to be crosslinked isrelatively slight, particularly on account of a high proportion ofelectrically conducting substance(s), and also on account of acomparatively minor actinic radiation output.

The object of the invention is also achieved by a process for thehardening of a polymer layer which, on account of the high pigmentcontent and its layer thickness, cannot be irradiated with actinicradiation through its whole layer thickness and therefore cannot bethrough-hardened by actinic radiation alone, wherein a UV hardening isfollowed by a chemical post-crosslinking, which may optionally beaccelerated or intensified by the action of heat, the polymer layersbeing largely or wholly through-hardened. The through-hardening layerthickness may in this connection be at least 5 μm and in particular upto 200 μm. The content of pigments may be at least 16 wt. %, preferablyat least 24 wt. %, in particular at least 32 wt. %, referred to thecomposition of the wet paint.

On account of the combination of UV hardening with chemical hardening,which may optionally be accelerated or intensified by the action ofheat, it is possible to through-harden completely or substantially overtheir whole volume coatings which, on account of the high pigmentcontent of the polymer coating and on account of its layer thickness,can no longer be irradiated through the whole layer thickness with UVradiation and therefore also cannot be through-hardened. The morestrongly the polymer layer absorbs the actinic radiation, in particularUV radiation, which may be associated with the content of pigmentparticles, the greater the need for chemical post-crosslinking. Thisapplies in a corresponding way the thicker the polymer layer.Accordingly polymer coatings with a layer thickness in the range from 5to 200 μm can now be hardened, which otherwise can only be partiallyhardened with actinic radiation without chemical post-crosslinking.

On account of the high degree of crosslinking the coating mixtureaccording to the invention exhibits, after the hardening, an improvedresistance to corrosive media such as for example salt solutions and toalkalis, and in particular on account of the isocyanate/isocyanuratesystem an improved resistance to organic solvents. The dry film hardenedaccording to the invention has a very high corrosion resistance alsowith low layer thicknesses. It exhibits a very good bondability betweenthe polymer-coated surfaces.

It appears that with this chemical system a weldable primer has beendeveloped for the first time that can be successfully used in massproduction, in particular in high-speed strip plants.

Surprisingly the need for UV radiation could already be reduced ininitial trials by 30 to 40% and subsequently by 60 to 70% by the use ofpost-crosslinking compounds. This indicates the high potential forreducing the strip speed. At the same time the hardened dry filmexhibits a high ductility, even though it was not primarily designed forforming operations.

The metallic or polymeric substrates coated according to the inventionmay be used in particular in the automobile, aircraft and aerospaceindustries, in building and construction, for external linings, rooflinings and in internal construction, in mechanical engineering andapparatus construction, for cabinet elements, shelving and domesticappliances.

According to the invention a wire, strip or part can be coated, whereinthe substrate to be coated is preferably a wire, a wire coil, a wiremesh, a steel strip, a metal sheet, a cladding/lining, a screening, acar body or a part of a car body, a part of a vehicle, trailer, mobilehome or missile, a covering, a housing, a lamp, a light, a hanging lightunit, an item of furniture or furniture part, a component of a domesticappliance, a frame, a profiled section, a moulded part of complicatedgeometry, a beam barrier, radiator or sauna part, an automobile bumper,a part made of or with at least one pipe and/or a profiled section, awindow, door or bicycle frame, or a small part such as for example ascrew, nut, flange, spring or a spectacle frame.

EXAMPLES AND COMPARISON EXAMPLES

The following examples, which are reproduced in the tables, illustratepreferred embodiments of the process according to the invention.

When formulating a composition the free-radically hardening compound wasfirst of all taken. All further additives were then added in a premixedstate (3. to 7.) and stirred. The pigments (8. to 11.) as well as thestabiliser for the suspension (12.) were then stirred in and dispersedwith a high-speed stirrer. This mixture was then ground in a bead millto a fineness of 5 to 20 μm. Finally the viscosity was adjusted with asmall addition of fully deionised water. If necessary the pH wasadjusted to the necessary value in the range from 8 to 9 by addingammonia. Following this the mixture was passed through a screen of 40 μmmesh width. The post-crosslinking compound (2.) was in each case addeddirectly to this mixture before the processing stage.

The application of the ready-for-use mixture provided with thepost-crosslinking compound was carried out on a chromate-free hot-dipgalvanised steel sheet pretreated with zirconium hexafluoride, using a 7μm doctor blade. The water content was evaporated at 60° C. over 10 to15 seconds and the film was then UV hardened with a UV-CK1 lamp and aUV-CK lamp at a strip speed of 40 m/min and at 160 W/cm lamp output perlamp. The layer thicknesses of the UV-hardened coatings were in therange from 2 to 10 μm. The post-crosslinking, which apart from theexamples involving capped post-crosslinking compounds took placeimmediately under the thermal influence of the UV radiation, was notaccelerated by additional thermal energy. The post-crosslinking of thecoating occurred in an intensified manner during the cooling of themetal sheets over ca. 24 hours, during which the substrate temperaturewas above 40° C. After 2 days the post-crosslinking was substantiallycomplete. After this time the metal sheets coated in this way were stillnot completely hardened. After storage for more than 10 days the metalsheets treated according to the invention were almost completelycrosslinked and exhibited the properties shown in Tables 2 and 3. Thecalculations are based on the composition of the wet paint and thereforeinclude water and possibly also organic solvent in the compositioncalculated as 100 wt. %. The dry film layer thickness was determinedgravimetrically.

TABLE 1 Composition of the mixtures of examples according to theinvention (B) and comparison examples (VB) with data in weight percentExample or Comparison Example B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 VB 1 VB 1 1. 42.5% polyurethane dispersion in water, 43.18 40.91 47.73 47.7337.50 35.00 30.00 32.50 55.00 30.00    free-radical hardening  2.Polyfunctional isocyanurate based on 4.54 4.54 4.54 4.54 4.00 4.00 3.004.00* — —    HDI (post-crosslinking) capped  3. 1-hydroxycyclohexylphenyl ketone 2.27 2.27 2.27 2.27 2.00 2.00 1.50 2.00 2.50 2.50  4.Bis(2,6-dimethoxybenzoyl)-2,4,4- 0.45 0.45 0.45 0.45 1.00 1.00 0.90 1.000.50 0.90    trimethylpentylphosphine oxide  5. Dipropylene glycolmonomethyl ether 1.82 1.82 1.82 1.82 2.00 1.40 0.25 1.40 2.00 2.00  6.Polyethylene-modified 0.45 0.45 0.45 0.45 0.50 0.50 0.25 0.50 0.50 0.25   polydimethylsiloxane  7. Polyether-modified acrylic-functional 0.450.45 0.45 0.45 0.10 0.10 0.10 0.10 0.50 0.10    polydimethylsiloxane  8.Iron phosphide 36.36 38.64 29.55 32.73 50.00 54.00 62.50 55.00 35.0060.00  9. Magnetic iron oxide 4.54 4.54 4.54 4.54 — — — — 2.50 2.50 10.Silicate pigment modified with calcium 1.36 1.36 1.82 — 2.00 2.00 1.502.00 — —    ions 11. Graphite 0.91 0.91 0.91 — — — — 0.50 — — 12.Hydrophobic silicic acid 0.23 0.23 0.23 0.23 — — — — 0.25 0.25 13.Water, added separately 3.41 3.41 5.23 4.77 0.90 — — 1.00 1.25 1.50*Identical isocyanurate as in Examples B1 to B7, but in addition capped

TABLE 2 Properties of the coatings according to the invention withvariations in the layer composition Properties B 1 B 2 B 3 B 4 B 5 B 6 B7 B 8 VB 1 VB 1 Dry film thickness in μm 6 6 6 6 6 6 6 6 6 6 Elasticityof the coating 7.6 7.8 9.2 8.8 7.4 7.0 6.5 7.4 7.3 6.0 in mm: cuppingtest according to DIN EN ISO 1520 Compression strength low low low lowhigh very high very high high Low high of the coating (Buchholzhardness) Hardness of the coating low low low low high very high veryhigh high Low high Formability on bending good good good good averageaverage poor good good poor over an edge to poor Defects after formingon crack- crack- crack- crack- slight marked cracks slight crack- cracksbending over an edge free free free free cracks cracks cracks freeElectrical conductivity of low Low very low low average average highaverage Low very high the coating Bonding properties, 50:50 50:50 50:5055:45 70:30 75:25 85:15 60:40 60:40 90:10 visually judged by the Schältest with epoxide resin bonder: adhesion-cohesion fracture Salt spraytest according unchanged unchanged unchanged marked slight marked markedslight marked marked to DIN 50021 SS, flat white white white white whitewhite white sample after 500 hours rust rust rust rust rust rust rustSalt spray test according average average slight marked slight some redsome red slight some red marked to DIN 50021 SS, flat white white whitered rust white rust rust white rust red rust sample after 1000 hoursrust rust rust rust rust Paint adhesion after slight slight slightmarked slight slight slight slight marked marked corrosion protectionpeeling peeling peeling peeling peeling peeling peeling peeling peelingpeeling with adhesive strip Resistance to alkaline very very very goodgood adequate satisfactory good unsatisfactory unsatisfactory cleaningagents good good good

TABLE 3 Properties of the coatings according to the invention withvariations in dry film thickness and proportions of crosslinking agentProperties B 9 B 10 B 11 B 12 B 13 B 14 Composition Mixture acc. to B3Mixture acc. to B7 Dry film thickness in μm 2 6 10 2 6 10 Propn.electrically conducting 37.5 37.5 37.5 62.5 62.5 62.5 particles, %Propn. UV hardening, % 80 80 80 80 80 80 Propn. of post-cross-linking, %20 20 20 20 20 20 Elasticity of the coating in mm: 9.6 9.2 7.1 7.2 6.54.9 cupping test according to DIN EN ISO 1520 Compression strength ofthe coating low low Low very very very (Buchholz hardness) high highhigh Hardness of the coating low low Low very very very high high highFormability on bending over an edge very good Satisfactory good poorpoor good Defects after forming on bending crack- crack- slight crack-cracks Cracks over an edge free free cracks free Electrical conductivityof the coating high very very very high High low low high Bondingproperties, visually judged 50:50 50:50 60:40 70:30 85:15 85:15 by theSchäl test with epoxide resin bonder: adhesion-cohesion fracture Saltspray test according to DIN slight unchanged Unchanged marked markedunchanged 50021 SS, flat sample after 500 hours white white white rustrust rust Salt spray test according to DIN some slight Unchanged somesome some 50021 SS, flat sample after 1000 red rust white red rust redrust white hours rust rust Paint adhesion after corrosion slight slightNo slight slight slight protection with adhesive strip peeling peelingpeeling peeling peeling peeling Resistance to alkaline cleaning veryvery very satisfactory satisfactory satisfactory agents good good goodProperties B 15 B 16 B 17 B 18 B 19 B 20 Composition Mixture acc. to B3Mixture acc. to B7 Dry film thickness in μm 6 6 6 6 6 6 Propn.electrically conducting 37.5 37.5 37.5 62.5 62.5 62.5 particles, %Propn. UV hardening, % 40 60 70 40 60 70 Propn. of post-cross-linking, %60 40 30 60 40 30 Elasticity of the coating in mm: 8.6 9.0 9.1 5.4 6.16.5 cupping test according to DIN EN ISO 1520 Compression strength ofthe coating high high fairly extremely extremely very (Buchholzhardness) low high high high Hardness of the coating high high fairlyextremely extremely very low high high high Formability on bending overan edge good good good poor Poor poor Defects after forming on bendingslight crack- crack- marked marked cracks over an edge cracks free freecracks cracks Electrical conductivity of the coating very low very lowvery low high High high Bonding properties, visually judged 50:50 50:5050:50 50:50 60:40 85:15 by the Schäl test with epoxide resin bonder:adhesion-cohesion fracture Salt spray test according to DIN unchangedunchanged unchanged unchanged unchanged slight 50021 SS, flat sampleafter 500 hours white rust Salt spray test according to DIN slightslight slight unchanged unchanged marked 50021 SS, flat sample after1000 white white white white hours rust rust rust rust Paint adhesionafter corrosion slight slight slight no no slight protection withadhesive strip peeling peeling peeling peeling peeling peelingResistance to alkaline cleaning extremely extremely very very good goodagents good good good good

The composition of Example 3 showed, in proportion to the content ofpolyurethane dispersion and on account of a content of anticorrosionpigment (silicate pigment), the best anticorrosion results. In additionExample 7 showed, on account of the largest proportion of electricallyconducting pigments (iron phosphide, magnetic iron oxide or graphite),the best electrical conductivity of the applied coating and thereforealso the best suitability for welding. In comparison example 1 nopost-crosslinking compound was added, with the result that the alkalineresistance was worst since free OH and polyol groups were still presentin the dried film. Although comparison example 2 shows the highestelectrical conductivity and thus the best suitability for welding, italso has the worst values as regards chemical resistance andweldability.

In the examples of Table 3 the chemical composition of the Examples B.3and B.7 were varied, on the one hand as regards the dry film layerthickness and on the other hand as regards the proportion of thecomponents that are responsible for the UV hardening or alternativelyfor the post-crosslinking. For Examples B.15 to B.20, which are basedonly partially on the composition of Examples B.3 and B.7, theproportion of the various types of crosslinking was varied by alteringthe proportions of free-radically polymerisable binders,post-crosslinking compound and photoinitiators proportionally to theproportion of the crosslinking in which they are involved. Withincreasing dry film thickness the welding suitability was significantlyreduced though the corrosion protection was significantly improved(Table 3). With increasing proportion of UV crosslinking the compressionstrength and hardness of the coating can be improved, though thecorrosion protection was reduced somewhat. Nevertheless the elasticityremained in the desired ranges.

In addition to the tests given in Tables 2 and 3 and their results, anMEK test was also carried out on the degree of crosslinking after thesecond day, after substantially complete hardening. Whereas in the caseof the metal sheets coated with the mixtures according to Examples 1 to4 about 40 double strokes with a cloth impregnated with methyl ethylketone were required and in the Examples 5 to 8 in each case 25 to 30double strokes were required in order to remove the coating, in the caseof the coating according to comparison examples 1 and 2 removal of thecoating was observed already after 5 to 10 double strokes.

In the salt spray mist test according to DIN 50021 over 500 and 1000hours, a very good corrosion resistance was observed in the metal sheetscoated with the mixtures according to the invention.

1. A mixture for the application of a polymeric coating to a substratecomprising: water, at least one water-dispersible free-radicallypolymerizable compound at least one water-dispersible post-crosslinkingcompound; at least one water-dispersible compound that forms freeradicals when irradiated with actinic radiation, wherein thefree-radically polymerizable compound does not completely harden at roomtemperature with the free radicals formed by the actinic radiation andcontains reactive bonds that in contact with the post-crosslinkingcompound lead to a major or complete hardening; and at least onesubstance selected from the group consisting of an electricallyconductive substance and waxes and a sufficient amount of silane orsiloxane to promote the slideability of the coating, wherein the mixturecomprises 10 to 50 wt. % water; 10 to 30 wt. % binder; 30 to 55 wt. % ofat least one electrically conducting substance; 1 to 20 wt. % of apost-crosslinking compound; and 1 to 60 wt. % of at least onephotoinitiator; calculated as composition of the mixture.
 2. A mixturefor the application of an anticorrosion layer to a metallic substrate,said mixture containing water, optionally an organic solvent; at leastone free-radically polymerizable binder; at least one compound formingfree radicals when irradiated with actinic radiation; at least oneelectrically conducting substance; at least one post-crosslinkingcompound; and at least one wax.
 3. A mixture according to claim 1,wherein free-radically polymerizable binder is water-dispersible and isselected from the group consisting of acrylates, methacrylates, epoxyresins, polyurethanes and polyesters, preferably from epoxy acrylates,urethane acrylates and acrylic-containing polyesters, in particular fromepoxy acrylates, urethane acrylates and acrylic-containing polyestersthat comprise free OH groups.
 4. A mixture according to claim 1,comprising at least one free-radically polymerizable compound thatcomprises at least one free OH or polyol groups.
 5. A mixture accordingto claim 1, wherein the free-radically polymerizable binder comprises atleast one group that is ethylenically, methylenically or propylenicallyunsaturated and is derived from a α-, β-unsaturated carboxylic acids. 6.A mixture according to claim 1, wherein the post-crosslinking compoundis selected from the group consisting of a water-soluble isocyanate, awater-soluble isocyanarate, a water-dilutable isocyanate, awater-dilutable isocyanarate, a water-dispersible isocyanate orisocyanurate based on TDI, MDI, HMDI or HDI, in particular based on HDI.7. A mixture according to claim 1, wherein the at least onepost-crosslinking compound is selected from the group consisting of anisocyanate or an isocyanurate and the free-radically polymerizablebinder is a urethane acrylate comprising hydroxyl groups.
 8. A mixtureaccording to claim 1, wherein said electrically conducting pigmentcomprises at least one material selected from the group consisting of aphosphide, phosphate or oxide of aluminum, iron, copper, or manganese,molybdenum-graphite, graphite-mica and magnetic iron oxide.
 9. A mixtureaccording to claim 1, wherein said electrically conducting particleshave at least one of a mean particle size or a maximum particle size asmeasured under a scanning electron microscope that is greater than theaverage dry film layer thickness of the layer produced therewith afterhardening.
 10. A mixture according to claim 1, comprising 10 wt. %water; calculated as composition of the mixture.
 11. A mixture for theapplication of a polymeric coating to a substrate comprising: water, atleast one water-dispersible free-radically polymerizable compound atleast one water-dispersible post-crosslinking compound; at least onewater-dispersible compound that forms free radicals when irradiated withactinic radiation, wherein the free-radically polymerizable compounddoes not completely harden at room temperature with the free radicalsformed by the actinic radiation and contains reactive bonds that incontact with the post-crosslinking compound lead to a major or completehardening; and at least one substance selected from the group consistingof an electrically conductive substance and waxes and a sufficientamount of silane or siloxane to promote the slideability of the coatingwherein said at least one substance is the electrically conductivesubstance. wherein the electrically conductive substance is present inan amount of from 30-75% by weight of the mixture.
 12. A mixture for theapplication of an anticorrosions layer to a metallic substrate, saidmixture comprising: water, at least one free-radically polymerizablebinder; at least one compound forming free radicals when irradiated withactinic radiation; at least one electrically conducting substance; atleast one post-crosslinking compound; and at least one wax; wherein theelectrically conductive substance is present in an amount of from 30-75%by weight of the mixture.
 13. A mixture for the application of apolymeric coating to a substrate comprising: water, at least onewater-dispersible free-radically polymerizable compound at least onewater-dispersible post-crosslinking compound; at least onewater-dispersible compound that forms free radicals when irradiated withactinic radiation, wherein the free-radically polymerizable compounddoes not completely harden at room temperature with the free radicalsformed by the actinic radiation and contains reactive bonds that incontact with the post-crosslinking compound lead to a major or completehardening; and at least one substance selected from the group consistingof an electrically conductive substance and waxes and a sufficientamount of silane or siloxane to promote the slideability of the coating,wherein said water-dispersible polymerizable post-crosslinking compoundis selected from the group consisting of isocyanate, isocyanurate,melamine resin and compounds that release at least one of isocyanate andisocyanurate at an elevated temperature.
 14. A mixture for theapplication of a polymeric coating to a substrate comprising: water, atleast one water-dispersible free-radically polymerizable compound atleast one water-dispersible post-crosslinking compound; at least onewater-dispersible compound that forms free radicals when irradiated withactinic radiation, wherein the free-radically polymerizable compounddoes not completely harden at room temperature with the free radicalsformed by the actinic radiation and contains reactive bonds that incontact with the post-crosslinking compound lead to a major or completehardening; and at least one substance selected from the group consistingof an electrically conductive substance and at least one wax, whereinthe mixture comprises 10 to 50 wt. % water; 10 to 30 wt. % binder; 30 to55 wt. % of at least one electrically conducting substance; 1 to 20 wt.% of a post-crosslinking compound; and 1 to 60 wt. % of at least onephotoinitiator; calculated as composition of the mixture.
 15. A mixturefor the application of a polymeric coating to a substrate comprising:water, at least one water-dispersible free-radically polymerizablecompound at least one water-dispersible post-crosslinking compound; atleast one water-dispersible compound that forms free radicals whenirradiated with actinic radiation, wherein the free-radicallypolymerizable compound does not completely harden at room temperaturewith the free radicals formed by the actinic radiation and containsreactive bonds that in contact with the post-crosslinking compound leadto a major or complete hardening; and at least one substance selectedfrom the group consisting of an electrically conductive substance andwaxes and a sufficient amount of silane or siloxane to promote theslideability of the coating, wherein said water-dispersiblepolymerizable post-crosslinking compound is selected from the groupconsisting of isocyanate, isocyanurate, melamine resin and compoundsthat release at least one of isocyanate and isocyanurate at an elevatedtemperature.
 16. A mixture for the application of an anticorrosion layerto a metallic substrate, said mixture containing water, optionally anorganic solvent; at least one free-radically polymerizable binder; atleast one compound forming free radicals when irradiated with actinicradiation; at least one electrically conducting substance; at least onepost-crosslinking compound; and at least one wax wherein saidwater-dispersible polymerizable post-crosslinking compound is selectedfrom the group consisting of isocyanate, isocyanurate, melamine resinand compounds that release at least one of isocyanate and isocyanurateat an elevated temperature.